Unmasking timekeeper of 24-hour cycle in mammals

Researchers at Harvard Medical School have identified what may be the key switch in mammals’ 24-hour clock: a simple protein that leads to the demise of its own genetic precursor. Their work provides interesting clues to a fundamental and longstanding mystery regarding a genetic mechanism known to drive this 24-hour biological clock, otherwise known as the circadian rhythm.

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Scientists believe that circadian rhythms operate on a series of interlocked genetic reactions that define what researchers call a feedback loop. However, the precise mechanism of this process—that is, how and what closes this genetic loop—has eluded scientists for decades. The answer, the researchers propose, may be a well known protein, one that plays a secret role in sustaining the ubiquitous circadian cycle.

“Our research solves a mystery about how the genetic feedback loop works in mammals,” said Charles Weitz, the Robert Henry Pfeiffer Professor of Neurobiology at HMS and senior author on a paper published June 17 in Science.

First described in the 17th century, the term circadian is derived from the Latin circa, meaning around, and diem, meaning day. Circadian clocks govern physiology and behavior in mammals (sleeping, waking and feeding, for example) and play a critical role that allows these animals to synchronize their metabolic cycles with the environment. Present in ancient life forms, including blue-green algae, such clocks are thought to have originated almost a billion years ago, possibly as a means of scheduling DNA replication for nighttime in order to protect genes from harmful solar radiation that penetrated the era’s thin atmosphere.

In the paper, Weitz, with lead authors Hao Duong and Maria Robles, and co-author Darko Knutti, all postdoctoral fellows at HMS, suggest that the key to the circadian riddle may be a novel function of a common protein known as Polypyrimidine tract–binding protein–associated splicing factor, or PSF.

Working with mouse models, Weitz and his team found that the protein PSF resides within a tangle of proteins called the PERIOD, or PER, complex. PER complexes are created by transcription factors of the PER gene. (A transcription factor is a protein that causes particular genes to be turned on or off.) The PER gene is activated by two other proteins residing on it: CLOCK and BMAL1.  Once turned on, PER protein has the ability to inhibit, or turn off, CLOCK and BMAL1, the very transcription factor that created it, closing the genetic loop for a certain period. This operation is crucial for maintaining a 24-hour clock in mammalian cells.

However, the PER protein cannot perform this operation unless it is carried to the site of CLOCK and BMAL1. The PSF protein within the PER complex plays the critical role of carrier: PSF protein carries the PER cargo back to CLOCK and BMAL1. The PER complex then turns the two proteins off for a limited time. “Without the presence of PSF, the whole loop ceases to function—or functions in ways that do not maintain the 24-hour clock,” said Weitz.

The series of genetic reactions takes roughly 24 hours. At the end of that time span, CLOCK and BMAL1 are turned on once again, and the circadian cycle begins anew.

A billion years after the circadian clock originated and centuries after scientists and philosophers learned of its existence, researchers finally have a mechanism that potentially explains how mammals maintain 24-hour clocks.

The work is far from over, however. “In fact, it has only just begun, now that we have a sense of the mechanism,” said Weitz. “There may be more PSF-like proteins.” Researchers in his laboratory are looking into potential functions of previously unknown proteins that may provide insights into the genetic interactions governing 24-hour mammalian circadian rhythms.

—Atreyee Bhattacharya